The invention concerns a sample head for flowthrough NMR spectroscopy with a measurement cell, an inflow tube through which an eluent flows into the measurement cell, an outflow tube through which the eluent coming from the measurement cell flows out, a measurement coil surrounding the measurement cell, and a vessel encasing and thermally shielding the measurement cell.
In flowthrough NMR spectroscopy, an eluent under pressure flows through a measurement cell that is located in a sample head in the magnetic field of the spectrometer, and from there flows back to a collection container.
FIG. 1 shows a schematic depiction of a known sample head for flowthrough NMR spectroscopy. Located in the interior of a glass cylinder 11, configured for example as a Dewar vessel 1, which has a cover 12 and a base 12', is a glass measurement cell 13, the interior of which has a predefined measurement volume. A radiofrequency measurement coil 16 is provided around the outer wall of the measurement cell 13. The coil 16 is generally configured as a saddle coil which generates in the vicinity of the measurement coil a radiofrequency field that runs perpendicular to the long axis of the glass cylinder 11, while the static magnetic field of the spectrometer is oriented along that axis. With flowthrough NMR spectroscopy, measurements can be made not only with continuously flowing eluent (continuous-flow process) but also when the eluent flow is discontinuous (stop-flow process).
To prevent, especially with the stop-flow process, any backmixing of the substance being measured, which might, for example, lead to smearing of the chromatographic peaks measured in the NMR spectrometer, a very thin inflow tube 14, having an inside diameter in the range 0.1 to 0.5 millimeter, is necessary. A glass measurement cell 13 that is drawn out at its lower end into such a capillary then has at that end only a very thin wall, which can break under very little force.
To prevent gassing with certain eluent mixtures, a certain back pressure on the order of 5 to 10 bars must be present.
With SFC (supercritical fluid chromatography) measurements, the equipment is operated at very high pressures, on the order of several hundred bars.
It is therefore necessary to provide a suitably insensitive, leakproof and pressure-stable joint between the inflow capillary tube and the measurement cell, and between the outflow capillary tube and the measurement cell.
Until now the approach to creating the aforesaid transition point has been to make do by shrinking narrow Teflon tubes of different diameters, in a stepwise fashion, onto the ends of glass measurement cells that are only slightly tapered.
However, such improvised transition points often fail to meet sealing requirements and also cannot withstand the pressures referred to earlier. But poorly sealed measurement cells yield inaccurate readings and result in damage to the sample head electronics due to organic solvents and water, and measurements often must be interrupted to replace the sample head.
A further disadvantage of this known sample head shown in FIG. 1 is the fact that the return section of the outflow tube 15 is located within the interior of the sample head vessel 1 and thus in the vicinity of the RF measurement coil 16. The material flowing out through the outflow tube 15 thus contributes to the reading, and can undesirably decrease its accuracy. If the measurement cell depicted in FIG. 1 needs to be replaced, for another measurement, by another measurement cell that, for example, has a different interior volume or a different wall thickness, the entire arrangement--consisting of the measurement cell and the inflow and outflow tube shrunk onto it--must be replaced. To do so, the RF coil must be unsoldered, the cover 12 and base 12' must be removed from the glass cylinder 11, and the inflow and outflow tube must be withdrawn from the base.